(1) Field of the Invention
The present invention relates to a rhodium catalyst containing supported rhodium metal omnipresent in the surface layer or in the neighborhood of the surface layer of the particles of a silica type or titania type support. The present invention also relates to a method for preparing a rhodium catalyst containing supported rhodium metal omnipresent in the surface layer or in the neighborhood of the surface layer of the particles of a silica type or titania type support.
(2) Description of the Prior Art
Catalysts containing noble metals, such as, platinum, palladium, rhodium and the like, supported on porous inorganic supports are heretofore widely used as catalysts for various reactions in the industry. In most reactions, the reactions substantially proceed in the outer surfaces or in the surface layers of the catalyst particles and the noble metals supported in the interior portions of the catalyst particles are not usually concerned with the reactions. Therefore, since the impregnation of the valuable and costly noble metals into the interior portions of the catalyst particles is not economical it is desired in the art that the noble metals are supported only in the surface or in the neighborhood of the surface of the support particles.
For instance, it is disclosed in Japanese Patent Publication No. 47-35670/1972 that a palladium catalyst containing palladium metal supported on the surface of support particles is prepared by impregnating the surface of support particles with a palladium salt after adjusting an aqueous acidic solution of the palladium salt to a pH of 2.8 to 4.8 by the addition of an alkali carbonate and, then, reducing the impregnated support particles. It is reported that the catalyst containing palladium supported only in the surface layer of the support has a high activity as compared with catalysts containing palladium substantially uniformly distributed in the entire portions including the interior portions of support particles.
Furthermore, it is also disclosed in Japanese Patent Publication No. 48-10135/1973 that vinyl acetate is prepared by using a catalyst containing 90% or more of palladium deposited on the surface of support particles and that excellent reaction results can be obtained by the use of this catalyst, as compared with the use of catalysts in which palladium is impregnated into the depths of support particles. It is reported that the catalyst containing 90% or more of palladium supported on the surface of the support is prepared by first depositing 0.001 to 0.2% by weight, based on the support, of an appropriate reduced metal (e.g. palladium metal) on the support and, then, depositing a necessary amount of a palladium catalyst component thereon.
Thus, it is already known in the art that the use of a catalyst containing a noble metal catalyst component supported in the surface layer or in the neighborhood of the surface layer of porous support particles results in a remarkably high catalytic efficiency (e.g. a yield based on the unit weight of the catalyst), as compared with the use of conventional catalysts having a catalyst component impregnated into the interior portions or the depths of the support particles. It is also known that the use of a catalyst containing a noble metal catalyst component supported in the surface layer or in the neighborhood of the surface layer of the support particles remarkably decreases the catalyst cost, due to the fact that the catalyst component contained in the interior portions of the support particles, which does not effectively catalyze the reaction, is not present.
Catalysts containing rhodium supported on porous supports are heretofore used in the synthesis of oxygen-containing compounds, such as, ethylene glycol, ethanol, acetaldehyde, acetic acid and the like, from synthesis gas (i.e. gas mainly containing hydrogen and carbon monoxide), in the purification of automotive exhaust gas, and various hydrogenation reaction, such as, for example, the selective hydrogenation of carbonyl groups of aldehydes, ketones and the like, in the nuclear hydrogenation of aromatic compounds, and in the hydrogenation of the unsaturated bonds of olefins, acetylene, nitriles and the like. Rhodium is a noble metal mainly derived from platinum ore. Therefore, the use of a catalyst containing rhodium substantially supported in the surface layer or in the neighborhood of the surface layer of support particles is also desirable from the point of view of not only the improvement in the catalytic efficiency, but also the decrease in the catalyst cost, as in the case of platinum and palladium catalysts. However, in the case where a silica type or titania type support is used, it is not possible to prepare a rhodium catalyst containing rhodium substantially supported in the surface layer or in the neighborhood of the surface layer of the support particles by using a conventional catalyst preparation method. This is also true when the above-mentioned methods for preparing the palladium or platinum catalysts containing the catalytic component substantially supported on the surface of the support particles are used.